Crazy about one girl (4)
I believe the answer is:
maid
'girl' is the definition.
(maid is a kind of girl)
'crazy about one' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'mad' going around 'i' is 'MAID'.
(Other definitions for maid that I've seen before include "Young woman - servant" , "Serving girl" , "Damsel" , "employee" , "- - -of-all-work" .)